Most of electronic devices need stable power supply. However, only a larger-size device, such as a personal computer or a further lager system, has a built-in power supply device. A smaller-size device or a portable device needs an adapter to receive power. One end of the adapter is connected to a city power, and the other end is connected to a connector to output a DC (Direct Current) power. Refer to FIG. 1 for a conventional adapter 90, which may be a mobile phone charger or a DC power plug for a small-size electric appliance. The adapter 90 comprises an AC/DC converter 91 and a DC power cable 92. The AC/DC converter 91 has at least one conduction terminals 911. The AC/DC converter 91 receives an AC (Alternating Current) power via the conduction terminal 911 plugged into a socket. Then, the AC/DC converter 91 converts the AC power into a DC power of a rated voltage and a rated current. The DC power is transmitted to an electronic device via an output terminal 93 at the end of the DC power cable 92. Refer to FIG. 2 for another conventional adapter 80, which may be a notebook computer adapter. The adapter 80 has an AC/DC converter 81, and two ends of the AC/DC converter 81 are respectively connected with an AC power cable 84 and a DC power cable 85. The AC power cable 84 has an input terminal 82 at one end thereof, and the DC power cable 85 has an output terminal 83 at one end thereof. The AC power cable 84 and DC power cable 85 may respectively have electromagnetic wave filters 86. The AC/DC converter 81 receives an AC power via the AC power cable 84. Then, the AC/DC converter 81 converts the AC power into a DC power. The DC power is output via the DC power cable 85. In the abovementioned conventional adapters, the DC power cable 85/92 between the AC/DC converter 81/91 and the output terminal 83/93 is used to transmit a lower-voltage DC power. For transmitting a given value of power, a lower-voltage DC power needs a higher current than a higher-voltage DC power and has a higher transmission loss than a higher-voltage DC power. The longer the DC power cable 85/92, the greater the transmission loss. Therefore, the length of the DC power cable 85/92 should be limited below a rational value in designing the adapter 80/90. Otherwise, the voltage of the output terminal 83/93 will be lower than the rated voltage because of transmission loss. However, the DC power cable 85/92 needs an appropriate length to transmit power from the AC/DC converter 81/91 to an electronic device. Thus, the DC power cable 85/92 has to adopt a high-quality cable to reduce the transmission loss of DC power. Then, the cost is increased. Those described above are the conventional problems the present invention intends to overcome.